A conventional stacking apparatus 10 is illustrated in FIG. 1. The stacking apparatus 10 is configured for use adjacent to a rotary die cut machine 12 which cuts blanks (not illustrated) from sheets of material, for example, corrugated paperboard. The stacking apparatus 10 includes a receiving or “layboy” section 14 that receives the blanks from the die cut machine 12 and discharges them onto a transfer conveyor 16. The transfer conveyor 16 carries the blanks to an inclined main conveyor 18, and the blanks travel along the main conveyor 18 to its downstream end 20 where they are discharged into a accumulator 22 (sometimes referred to as a “hopper”).
After the blanks are discharged from the downstream end 20 of the stacker, they impact a backstop 24 that defines a downstream end of the accumulator 22 and fall onto a receiving conveyor 26 mounted on a lift table 28. Side tamping elements 34 mounted on an intermediate rail 36 and/or front and rear tamping elements (not illustrated), which may be pneumatically powered, for example, repeatedly press inwardly on the stack from opposite sides and/or from the front and the rear, to align or square the stack of sheets. As the stack 30 on the lift table 28 grows, the lift table 28 drops, either continuously or periodically, so that the sheets are always falling approximately the same distance from the downstream end 20 of the stacking apparatus 10 onto the lift table 28 or onto the partial stack 30 on the lift table 28. Alternately, the stack may fall onto a fixed-height conveyor and the end of the main conveyor may rise to maintain a constant distance from the top of the stack.
The position of the backstop 24 and the positions of the side tamping elements 34 are based on the size of the sheets to be processed and on the number of rows or streams of sheets that will be simultaneously discharged from the downstream end 20 of the stacking apparatus 10. The position of the backstop 24 is generally set manually before each product run by loosening fasteners (not illustrated) and manually sliding the backstop toward and away from the downstream end 20 of the stacking apparatus 10 until it is located at a suitable position for receiving incoming sheets, a position that is, for example, about 10% further from the downstream end 20 of the stacking apparatus 10 than the length of the sheets that will be received in the accumulator 22. The fasteners are then tightened to hold the backstop 24 at this position. Likewise, the connections of the side tamping elements 34 to the intermediate rail 36 are loosened, and the side tamping elements 34 are positioned as necessary for a given product run. The side tamping elements 34 are configured to move linearly back and forth a certain distance, one to two inches for example, and are therefore positioned such that, when this limited motion occurs, the side tamping elements will press inwardly against a stack of sheets to square the stack.